This invention is directed toward geophysical measurement apparatus and methods employed during the drilling of a well borehole. More specifically, the invention is directed toward a compensated ensemble crystal oscillator clock system that comprises a plurality of quad compensated clocks. Physical fabrication of elements of the oscillator ensemble and cooperating data processing software minimize drift in frequency of the oscillator clock system in harsh borehole environments encountered while drilling the borehole. The oscillator clock system is particularly applicable to seismic-while-drilling operations wherein precise downhole measurements of time are required typically over a period of days.
Systems for measuring parameters within a well borehole, and geophysical parameters of earth formation in the vicinity of the well borehole, typically fall within two categorizes. The first category includes systems that measure parameters after the borehole has been drilled. These systems include wireline logging, tubing conveyed logging, slick line logging, production logging, permanent downhole sensing devices and other techniques known in the art. The second category includes systems that measure formation and borehole parameters while the borehole is being drilled. These systems include measurements of drilling and borehole specific parameters commonly known as “measurements-while-drilling” (MWD), measurements of parameters of earth formation penetrated by the borehole commonly known as “logging-while-drilling” (LWD), and measurements of seismic related properties known as “seismic-while-drilling” or (SWD).
Some measurements made while drilling a borehole require that sensor responses to parameters and events within the borehole, or “downhole”, be synchronized to sensor responses and events at the surface of the earth. As an example, SWD systems typically require the accurate and precise measure of travel time of acoustic energy between the surface of the earth and a downhole assembly disposed at a known depth within the borehole. The response of a sensor to a displaced source of acoustic energy is combined with synchronized outputs of cooperating clocks to obtain the measure of travel time. This measure of travel time is subsequently used to convert acoustic impedance interfaces measured in a time domain to corresponding positions of the interfaces in a depth domain. A “downhole” clock disposed within a downhole assembly is typically synchronized with a surface or “reference” clock disposed within surface electronics equipment. The downhole assembly is then deployed within the borehole. The downhole assembly can be deployed for a period of several days with no continuous communication link, such as an electrical cable, with the surface equipment.
In order to obtain the desired accuracy and precision, SWD measurement techniques typically require downhole clock and reference clock synchronization of the order of 1 millisecond (ms), over a time period as great as several days. As an example, drift of a downhole clock is preferably equal to or less than about 3 parts per billion (ppb) in order to maintain clock synchronization with a reference clock (preferably calibrated in Coordinated Universal Time or “UTC”) of about 1.0 ms or less over a period of about four days. Clocks that achieve this accuracy are available for use at room temperature. No clocks are known which provide the required accuracy in a borehole drilling environment, which typically involves elevated and varying temperature, significant shock, oscillator aging, high vibration, and other environmental conditions which adversely affect clock performance.